In semiconductor device processing so-called "native" oxide films form on freshly etched silicon due to exposure in air, water or the etching chemicals themselves. Typically, this "native" or "chemical" oxide is 5-15 .ANG. thick and contains all sorts of species in addition to SiO.sub.2, such as C, H, F, S, N, and other metals.
In typical deposition processes, a thermal oxide is formed in a pattern on the silicon substrate for masking the covered substrate portions to permit treatment such as metal deposition in the remaining exposed surface. In removing the mask material after the thermal oxide is formed a native oxide has been formed. It is desirable to remove this native oxide prior to metal deposition without removing any or removing as little as possible of the adjacent thermal oxide. Since it has not been possible in the past to control oxide etching at levels such as 5-15 .ANG. absolutely, it is generally the practice to over-etch. Thus, as much as 50 .ANG. to 100 .ANG. of the thermal oxide is etched, and often this amount of thermal oxide removal cannot be tolerated.
Most of the cleaning processes used today in the fabrication of semiconductors involve wet cleaning which leave or even cause native oxides which tend to degrade subsequent device properties.
P. J. Holmes and J. P. Snell "A Vapor Etching Technique for the Photolithography of Silicon Oxide", MicroElectronics and Reliability, Vol. 5, pp. 337-341, 1966, describe the use of vapor from a HF and H.sub.2 O solution as the etchant for the photolithographic stage of planar silicon device technology. Similar vapor etching using HF from an aqueous solution is described by K. D. Beyer by himself or with others in the IBM Technical Disclosure Bulletin, "Etching SiO.sub.2 in Gaseous HF/H.sub.2 O", Vol. 19, No. 7, p. 2513, December 1976, "Silicon Surface Cleaning Process", Vol. 20, No. 5, pp. 1746-1747, Oct. 1977, and "Removal of Native Oxide Layer on a Semiconductor Surface", Vol. 22, No. 7, p. 2839, Dec. 1979, and also reported in U.S. Pat. No. 4,264,734.
Two recent articles describe work in Japan on selective etching of native oxide using anhydrous HF. These are N. Miki, H. Kikuyama, M. Maeno, J. Murota, and T. Ohmi "Selective Etching of Native Oxide by Dry Processing Using Ultra Clean Anhydrous Hydrogen Fluoride," Tech. Dig. of International Electron Devices Meeting, pp. 730-733, San Francisco, DEC, 1988, and N. Miki, H. Kikuyama, I. Kawanabe, M. Miyashita and T. Ohmi "Gas-Phase Selective Etching of Native Oxide", IEEE Transaction on Electron Devices, Vol. 37, p. 107, 1990. These articles set forth that thermal oxide does not react with HF gas below the critical level of HF concentrations less than 4.7 vol. % in N.sub.2 flow and native oxide is selectively etched away in HF concentrations in the range of 0.06 to 4.7%. The authors maintain that mixtures of HF and water vapor are too corrosive to gas supply systems and process chambers to be practical for cleaning. No differences are noted with respect to the time delay between initiation of etching native oxide and initiation of etching of thermal oxide.
Several dry etching techniques of thermal oxides have been described in the literature using hydrogen fluoride vapor. The article "The Dry Process for Etching Silicon Dioxide" by R. L. Bersin and R. F. Reichelderfer, Solid State Technology, Vol. 20, pp. 78-80, April 1970, and U.S. Pat. No. 4,125,437 by the same authors describe the use of ahhydrous hydrogen fluoride for etching silicon dioxide without damaging the underlying layer of silicon. In the article "HF Vapor Phase Etching: Production Viability for Semiconductor Manufacturing and Reaction Model" by D. F. Weston and R. J. Mattox, J. Vac. Sci. Technol., Vol. 17, No. 1, pp. 466-469, Jan./Feb. 1980, a non-plasma silicon dioxide etch process using anhydrous hydrogen fluoride at reduced pressures is described involving the interaction of HF vapor with negative photoresist which catalyses subsequent etching beneath the photoresist. Ma Jun-Ru et al. in "A New Conformal Dry-Etch Technique for Submicrometer Structures", J. Vac. Sci. Technol., Vol. 19, No. 4, pp. 1385-1389 describe a technique involving diffusion of gaseous HF and water vapor through an exposed resist for selective etching of the underlying SiO.sub.2.
Use of anhydrous HF etching of native SiO.sub.2 has been described in work by FSI International, Inc. Publications include R. E. Novak, "Anhydrous HF Etching of Native SiO.sub.2 : Applications to Device Fabrication", Solid State Technology, pp. 39-41, March 1988 and U.S. Pat. No. 4,749,440.
U.S. Pat. No. 4,695,327 describes the method of treating a semiconductor wafer by a solvent introduced into a chamber until its pressure is close to but below the vapor pressure of the liquid solvent at the temperature in question so that a plurality of molecular layers of solvent are absorbed in microrecesses and thereafter increasing the vapor pressure until the solvent condenses and wets the microrecesses. This patent suggests the use of hydrogen fluoride gas to remove the native oxide on a semiconductor surface and the use of a mixture of gaseous hydrogen chloride, gaseous water and gaseous hydrogen peroxide to remove metals that can be complexed and solvated from hydrophobic semiconductor materials.